1. Field of the Invention
The present invention relates to an optical recording medium referred to as a next generation DVD (digital versatile disc) and, in particular, to a rewritable phase-change optical recording medium having two or more information layers formed of a phase change material.
2. Description of the Related Art
Blu-ray (trademark) discs (hereinafter abbreviated as BDs), for example, have been proposed as next generation DVDs. For BDs, an optical system is used which includes a recording-reproduction laser of a wavelength of 405 nm (blue) and an objective lens having a numerical aperture NA of 0.85 (λ/NA≦500 nm).
In optical disc drives for BDs or other discs, a single-mode oscillation diode laser is used, and high frequency modulation is applied to a reproduction laser beam at a frequency of several hundred MHz in order to reduce laser noise generated during signal reproduction. Generally, the high frequency modulation is performed under the conditions of a frequency of 300 to 500 MHz, a Ratio of 3 to 8, and a pulse width of 200 to 400 psec. Here, the “Ratio” is the ratio of the peak value of the laser power to the average value.
For BDs, rewritable optical recording media have been proposed which have two or more information layers on one side. In such multi-layer optical recording media, information layers other than an information layer (L0 layer) located farthest away from a laser beam incident surface must be semi-transparent information layers that are semi-transparent to the wavelength of a recording-reproduction laser beam, in order to allow the laser beam directed to the L0 layer to pass therethrough. For example, in dual-layer optical recording media, an L1 layer located on the light incident side of the L0 layer is a semi-transparent information layer. Therefore, the reflectivity from the L1 layer becomes low, and the reflectivity from the L0 layer also becomes low because the laser beam is incident on and reflected from the L0 layer through the L1 layer. In such dual-layer optical recording media, the reflectivity is lower than that of single-layer optical recording media. Accordingly, the power of the laser beam during reproduction (the reproduction power) must be increased to ensure a sufficient amount of light returning to a pickup.
Moreover, the laser beam used for BDs has a shorter wavelength than that used for DVDs, and an objective lens having a high NA is used in the BDs, whereby a spot size is reduced. Therefore, the energy density at the laser spot is very high.
Furthermore, as described above, since the L1 layer in the dual-layer optical recording media must be a semi-transparent information layer, the thickness of a metal reflection film must be reduced. Therefore, the heat generated by the laser beam irradiated onto the recording film of the L1 layer is not sufficiently dissipated from the reflection film, so that the cooling rate in the L1 layer is lower than that in the L0 layer, i.e., the L1 layer has a slow-cooling structure.
Moreover, when the linear velocity for recording is increased to perform high-speed recording, i.e., when the rotation speed of the disc is increased, the pickup cannot easily follow grooves on the disc, and therefore servo control is not stabilized. Hence, when high-speed recording is performed, the reproduction power must be increased to stabilize the servo control.
Therefore, in the semi-transparent information layers in next generation DVDs having two or more information layers, the reproduction beam causes deterioration of recorded signals, so that the reproduction durability is reduced significantly. This problem is caused by the following four main reasons: high reproduction power, high energy density of the laser spot, the slow-cooling structure in the semi-transparent information layers, and high-speed recording.
As described above, high frequency modulation is applied to the reproduction beam. Therefore, the higher the Ratio in the high frequency modulation, the higher the peak power of the reproduction beam, so that the temperature of the recording film in the portion irradiated with the reproduction beam increases significantly. Therefore, since a high-power reproduction beam under high frequency modulation at high Ratio is used in BDs, it is more difficult to achieve sufficient reproduction durability in the BDs than in DVDs.
In particular, in rewritable phase-change optical recording media, amorphous marks serving as recorded signals can be crystallized when irradiated with a laser beam having a high-reproduction power. In such a case, the recorded signals are likely to be lost, and durability of reproduction is significant.
In order to solve the above problems, Japanese Patent Application Laid-Open No. 2004-306595 discloses information recording media in which an Sb—Ge—In based alloy is used as the phase-change recording material, and Japanese Patent Application Laid-Open No. 2006-315242 discloses phase-change optical information recording media in which an Sb—Te based alloy is used as the phase-change recording material.